Integrated services digital network (ISDN) is a relatively newly developed and emerging field of telecommunications which integrates computer and communications technologies to provide, world wide, a common, all-digital network. This is based, in part, on standardizing the structure of digital protocols developed by the International Telegraph and Telephone Consultative Committee (CCITT) so that, despite implementation of multiple networks within national boundaries, from a user's point of view there is a single, uniformly accessible, worldwide network capable of handling a broad range of telephone, data and other conventional and enhanced services.
A complete description of the architecture of ISDN is beyond the scope of this specification. For details, and for an extensive bibliography of references on ISDN, see Stallings, ISDN, AN INTRODUCTION, Macmillan Publishing Company, 1989.
As an overview of ISDN, and the interfaces therein, reference is made to FIG. 1 wherein a customer premises is interconnected with a local exchange. At the customer premises an "intelligent" device such as a digital PBX, terminal controller or local area network (LAN) can be connected to an ISDN terminal TE, such as a voice or data terminal, which is connected, over an "RS-232-Interface", to a network termination NT1. Although not shown, non-ISDN terminals may be connected to a termination NT2 over the RS-232-Interface and another device termed a "terminal adapter". The NT2 in turn is connected over an "S/T-Interface", which is a four wire bus, to a termination NT1 that performs functions such as signal conversion and maintenance of the electrical characteristics of the loop. The NT1 thus is a termination between customer and telephone company equipment.
At the local loop, a two-wire bus, termed the "U-interface", or "loop" interconnects network termination NT1 and a loop termination LT at the central office. Finally, the "V-interface" is a bus between the local loop at the carrier end and exchange switching equipment. Details of this architecture are provided in Integrated Services Digital Networks (ISDN): An Overview, DataPro Research, Concepts & Technologies, MT20-365-101 to 110, published by McGraw Hill Incorporated (December, 1988).
A number of communication channels are established between the central office and ISDN subscriber; the transmission structure consists of a pair of B-channels each carrying 64 kilobits per second of data and a D-channel that carries 16 kilobits per second of data. The B-channels in practice are used to carry digital data, pulse code modulated encoded digital voice or a mixture of lower rate traffic including, optionally, packet switched data. The D-channel carries common channel signalling information to control circuit switched calls on associated B-channels at the user interface, and may also carry packet switching or low-speed telemetry. Data on the D-channel provides information to the central office switch on status of the customer telephone, e.g., that the customer has gone off-hook, and information necessary to control telephone functions such as activate the status lamp, control the ringer, etc.
Standards for the S/T-Interface have already been defined by CCITT; equipment supplied by different manufacturers which subscribe to those standards can be connected to network terminations as there will be no protocol incompatibilities. Standards for the U-interface, however, have not yet been defined. Special integrated circuits currently produced only by American Telephone and Telegraph (AT&T) and incorporated into AT&T equipment, are required for compatibility with the U-interface. Accordingly, whereas the principles of the present invention have general applicability to ISDN network testing, the invention shall be described in an AT&T system operating environment.
Thus, referring to FIG. 2, at the central office of the carrier (telephone company) is a solid state switch 20, provided by AT&T under the name 5ESS (TM) switch, for routing calls. As the 5ESS (TM) switch is well known, details will not be provided herein. An overview of digital switches is given in Switching Systems, DataPro Research Corporation, Concepts and Technologies, MT20-050-201 to 215, published by McGraw Hill Corporation (February 1988). Also at the central office is a conventional integrated services line unit (ISLU) 22, manufactured by AT&T, to interface customers with the 5ESS (TM) switch. The ISLU 22, which satisfies ISDN interface requirements, implements a "2B+D" channel structure and is compatible with both the T- and U-interfaces. The ISLU 22 receives up to 512 customer lines in 16 line groups, as shown in FIG. 3, and carries out duplex operation in two service groups at control cards CC0 and CC1. Data handling is carried out by cards CD0 and CD1, and metallic functions are performed by HV1 and HV2 which also contain line testing and high level service circuitry. Data cards CD0, CD1 are in circuit with the switch 20 over the standard peripheral interface data buses (PIDB) and directly connected PIDBs (DPIDB). The common control cards CC0, CC1 receive instructions from a central office switch processor (not shown) over the PICBs. The high voltage circuits HV0, HV1 are connected on the standard metallic test bus (MTB) to enable any metallic functions such as ringing of customer lines and line testing to be performed. A complete block diagram summarizing the architecture of the standard AT&T ISLU is shown in FIG. 4. The manner by which the AT&T ISLU processes customer calls originating with or terminating at the ISLU, is well known in the industry and shall not be described in detail herein.
Referring to FIG. 2, supplied to the ISLU 22 is the U-interface, used in ISDN, as well as the standard analog interface. Data on the U-interface is optionally monitored by a U-interface monitoring system (UIMS) 24, manufactured by AT&T, interposed in series between the ISLU 20 and a network termination NT1 associated with the telephone or terminal of an ISDN customer. The purpose of the UIMS 24 is to monitor the protocol of control data on the U-interface, to determine whether the ISDN line of a customer is functioning properly. The UIMS separates the B1-, B2-, and D-channel bit streams (2B+D) on the U-interface, and presents this data to a protocol analyzer 30 that typically is transported to the central office by service personnel.
The protocol analyzer 30, which is conventional, carries out ISDN protocol decoding to enable both B-channel and D-channel traffic to be tested. However, it is only the D-channel traffic that ordinarily is tested, as it is only the D-channel that contains the supervisory data necessary to assess the operation of a customer ISDN line, including information to control operations such as feature activation, lamp operation, ringing, dialing and supervision. In this regard, a protocol analyzer is analogous to a "butt-set" currently used to test analog telephone lines.
However, implementing efficient protocol analysis in an ISDN network having a large number of nodes poses particular problems. The prior art has sought to test ISDN networks by installing dedicated protocol analyzers to monitor the S/T-Interface; this is costly and time consuming as it requires a field visit on every trouble call. Furthermore, service personnel employed by the telephone company cannot properly access the customer side of the S/T-Interface.
Protocol monitoring and analysis are better performed at the central office. The test system shown in FIG. 2 implements the UIMS to monitor the customer ISDN lines at each central office. However, this also is disadvantageous because it still requires dispatching of service personnel to those central offices which are unmanned, as is necessary to physically insert a plug to interconnect the UIMS and the main distribution frame of the ISLU. Such a procedure is not easily implemented.
One particular system implemented in the prior art multiplexes the D-channel data obtained from the UIMS up to 56 kilobits per second or 64 kilobits per second, using a Remote Access Test Device (TM), manufactured by Tekelec, Calabasas, Calif., with input to one B-channel of an on-site standard 750.times. modem, to be transmitted to a central test facility for protocol analysis of a line under test. However, this technique requires that service personnel be dispatched to the central office to make the connection between the UIMS and main distribution frame, to access the U-interface bus.